Fix serialization of Graph.graph
Add edge costumes

This uses the primary sprite costume. The default K5 costume reverts
it to the usual black arrows. The UI... may need some tweaks.
Fix strong connectivity

Closes #79.
Add basic limits on number of visible nodes

TODO: WordNet loading and add node, generator checks.
Suggest displaying a subgraph in too-many-nodes message
Fix loading graph from URL not adding in attributes
Add WordNet loading
Prevent some mysterious slowdowns due to bound undrawn graphs leaking
React to change in max number of nodes by hiding/showing graph
Handle too many nodes case for generators
Fix off-by-one error in setGraphToDisplay2()
Actually hide graph after loading WordNet
Handle too many nodes case for add node

Closes #69 for real this time, I hope.
Add OpenStreetMap extracts
Add example which loads OSM data

Closes #77.